1. FIELD OF THE INVENTION
The present invention relates to a light-detecting device for converting a light to an electrical signal, and more particularly to a light-detecting device which internally multiplies charges by making use of an effect of an externally applied electric field to enhance a sensitivity.
The light-detecting device of the present invention includes one which converts an incident light to an electrical signal and multiplies charges by making use of an electric field effect by an externally applied electric field to enhance a sensitivity. Such photo sensitive device may include an avalanche photo-diode (APD), a photo-multiplier tube, an image intensifier type image pick-up tube, and a photo-conductive image pick-up tube.
2. RELATED BACKGROUND ART
In a prior art light-detecting device such as a photo-conductive image pick-up tube disclosed in JP-A-49-24619, a blocking type target is used to block charge injection from a signal electrode and scanning electron beam side. This type of light-detecting device has an advantage of small lag. However, in image pick-up tube having a blocking type target which has a gain larger than unity and a high sensitivity has not been known.
An image pick-up tube which uses an injection type target in which charges are injected from both or one of the signal electrode and the scanning electron beam has been known. In such an image pick-up tube which uses the injection type target, it is possible, in principle, to take out more carriers (electrons) than the number of incident photons to an external circuit and hence an image pick-up tube having larger than unity gain can be provided.
As such image pick-up tubes which uses the injection type targets, a high sensitivity image pick-up tube having a target made of an np single crystal (JP-B-43-18643) and a high sensitivity image pick-up tube having an electron injection recombination layer arranged on a scanning beam side of a photo-conductive layer to allow injection of scanning electrons and recombination of the scanning electrons and holes (1985 National Convention Record of the Institute of Television Engineers of Japan, pages 25 and 26) have been proposed.
However, in such a light-detecting device having the gain increased larger than unity, the sensitivity is highly sensitive to a surrounding environment even if the externally applied voltage is always kept constant. In the photo-conductive image pick-up tube which uses the blocking type target, a portion of the scanning electrons is injected into the target. Accordingly, an effective storage capacity of the target increases, in principle, and the lag increases.